A variety of devices are known for the fixation, internally or otherwise, of bone bodies in humans and animals. In the case of the internal fixation of the vertebrae of a spinal column, so that the vertebrae are maintained in a desired spatial relationship with respect to one another, the devices often include pedicle screw, or bone fastener, assemblies. These assemblies include a pedicle screw, or bone fastener, that is anchored, typically by a threaded arrangement, into the pedicle of each of the vertebrae that are to be maintained in a desired spatial relationship. One or more appliances such as longitudinal supports, which may comprise plates or rods, that extend longitudinally of the spinal column are connected securely to the pedicle screw assemblies in a manner that allows the vertebrae to be maintained in a desired alignment. In order to achieve the desired stability, the bone fasteners must be attached securely to the vertebrae and connected firmly to the appliance.
The bone fasteners may be connected directly to the rods or plates, or the connection between the bone fasteners and the rods may be accomplished by the use of a connector or coupling member that forms a part of the appliance. In the latter instance, the end of the bone fastener that is not attached to the vertebrae is secured to one end of the coupling member and the other end of the coupling member is secured to the rod or plate.
The secure placement of a rod with a rigid bone fastener, whether or not a coupling member is used, or the placement of a plate between two or more rigid screws is difficult for a variety of reasons. For example, the displacement or angulation of the vertebrae may be such that the rigidity of the bone fastener and/or coupling member, where a coupling member is used, prevents a secure connection from being made among the components of the bone alignment system. Additionally, installation is difficult because the rod or plate must align precisely with all of the bone fasteners.
Although a bend can be made in the rod or plate in order that the bone fasteners be firmly secured to the rod or plate, the possibility exits that stresses can be created that cause the bone bodies or vertebrae to fracture or the screws to loosen over time. This is even the case when rods having a degree of flexibility are used. Thus it is important that bone fastener assemblies be provided that minimize the likelihood of the establishment of undesirable stresses.